1. Field of the Invention
The present invention relates to a novel magnetic recording medium. More particularly, it relates to a magnetic recording medium showing excellent recording density and S/N ratio, causing no environmental pollution because of the absence of generation of hydrochloric acid on incineration for disposal after finishing the use and favorably utilized for magnetic tapes, magnetic sheets, magnetic discs and the like products.
2. Description of the Prior Art
Magnetic recording media are utilized in audio instruments, video instruments, computers and the like instruments and the demand for the magnetic recording media has been growing remarkably. The magnetic recording media generally have a structure comprising a non-magnetic supporter, such as polyester film, and a magnetic layer comprising magnetic powder and a binder and coated on the supporter. The magnetic layer is generally formed by coating or transferring a magnetic coating material comprising the magnetic powder dispersed in a medium containing the binder on the surface of the supporter.
As the binder in the magnetic recording media, organic macromolecular compounds, such as polyester resins, cellulose resins, polyurethane resins, polyvinyl chloride resins, vinyl chloride-vinyl acetate copolymers, vinyl chloride-vinyl acetate-vinyl alcohol copolymers, vinyl chloride-vinylidene chloride copolymers, acrylonitrile-butadiene copolymers, phenolic resins, epoxy resins, polyamide resins, polyvinyl butyral, nitrocellulose, cellulose acetate butyrate, acrylic resins, resins curable by electron beam and the like, are generally utilized.
Each of these binders has its own strong points and weak points. The binders are generally utilized as a combination of two or more kinds because a magnetic layer having a desired property is not usually obtained by a single kind of the binder. For example, a combination of a relatively hard resin, such as a polyvinyl chloride resin, polyvinyl butyral, nitrocellulose and the like, and a relatively soft resin, such as a polyester resin, a polyurethane resin, an acrylonitrile-butadiene copolymer and the like, is widely utilized. A polyisocyanate compound is also widely utilized as a hardening component in the binder for the purpose of increasing durability of the magnetic layer.
The ferromagnetic powder is progressively having a finer structure, a higher magnetic strength and a higher reluctivity in accordance with the requirements to increase the recording density and to improve the S/N ratio of magnetic recording media. The dispersing ability of the binder is, therefore, a very important factor for achieving the improvement of the properties of the magnetic recording media by dispersing the ferromagnetic powder uniformly in the magnetic coating material to form a highly filled magnetic layer having a smooth surface.
A dispersant, such as a low molecular weight surface active agent, is generally added for improving the dispersion of the ferromagnetic powder. When a large amount of the dispersant is used, decrease of durability of the magnetic recording media, soiling of the recording head and other undesirable phenomena take place and the amount of the dispersant is inevitably limited. Thus, the satisfactory dispersion can not be achieved by relying on an added dispersant and it is required that the binder have an intrinsic dispersion ability of a high degree.
It is generally practiced in the field of video magnetic recording tapes that the magnetic layer is crosslinked by compounding a hardening component like a polyisocyanate compound into the magnetic coating material for the purpose of increasing the durability and the reliability of the magnetic recording media. In this method, the binder is required to have reactivity of a suitable degree with the crosslinking agent like a polyisocyanate compound.
As the binder satisfying these conditions, polyvinyl chlorides resins comprising a hydrophillic group containing sulfur or phosphorus, such as a salt type strong acid group and a tertiary ammonium salt group of the formulae: SO.sub.3 M, SO.sub.4 M, PO.sub.4 M.sub.2 and PO.sub.3 M.sub.2 wherein M is an alkali metal or ammonium group, are widely utilized because of excellent dispersion ability and dispersion stability for the ferromagnetic powder and high strength based on strong intermolecular force.
Along with the growth of usage of the magnetic recording media, the amount of the magnetic recording media which finished its life of service is also increasing. When securing of secret information is considered, incineration is desirable as the method of disposal of such used magnetic recording media. However, when a polyvinyl chloride resin is used as the binder, hydrochloric acid gas is generated during the incineration and acidic mist may be discharged in the air if the incineration furnace is not equipped with an apparatus for discharge gas treatment. Of course, a method like the incineration with addition of lime powder can be adopted but the method has a problem that the amount of ash after the incineration is inevitably increased.
A binder containing no chlorine may be utilized to avoid the generation of hydrochloric acid gas during the incineration. Nitrocellulose resins, acrylate resins and styrene resins have been known as such a binder containing no chlorine. However, nitrocellulose resins are essentially dangerous because of the possibility of denitration and explosion and have insufficient ability as the binder for a magnetic recording media of higher recording density because of high viscosity of the magnetic coating material. It was proposed that the dispersion can be improved by introducing a sulfonic acid (salt) group or phosphoric acid group into acrylic resins or polystyrene resins (Japanese Laid Open Patent Application Showa 57-138050, ibid. Showa 57-138051, ibid. Showa 57-141020, ibid. Showa 58-108032 and Japanese Patent Publication Showa 62-7606). However, a larger alkyl group must be contained in the polymer to improve the dispersion ability by increasing the hydrophobic property of the backbone polymer and this structure brings a lower softening point to cause an unfavorable effect on the running property and the durability of the magnetic recording media. Polystyrene resins have an intrinsic problem that the resins have a weaker intermolecular force than polyvinyl chloride resins and the magnetic layer prepared is more fragile. Various methods have been tried to improve the fragility, such as copolymerization of a monomer containing hydroxyl groups with a styrene monomer, mixing with polyurethane resins and crosslinking by adding a polyisocyanate compound to these materials. Nevertheless, the running property and the durability of the binders based on polystyrene resins still remain inferior to those based on polyvinyl chloride resins.